As the Bush administration prepares to attack Iraq this week, it is doing so on the basis of a number of allegations against Iraqi President Saddam Hussein that have been challenged -- and in some cases disproved -- by the United Nations, European governments and even U.S. intelligence reports.

For months, President Bush and his top lieutenants have produced a long list of Iraqi offenses, culminating Sunday with Vice President Cheney's assertion that Iraq has "reconstituted nuclear weapons." Previously, administration officials have tied Hussein to al Qaeda, to the Sept. 11, 2001, terrorist attacks, and to an aggressive production of biological and chemical weapons. Bush reiterated many of these charges in his address to the nation last night.

But these assertions are hotly disputed. Some of the administration's evidence -- such as Bush's assertion that Iraq sought to purchase uranium -- has been refuted by subsequent discoveries. Other claims have been questioned, though their validity can be known only after U.S. forces occupy Iraq.

In outlining his case for war on Sunday, Cheney focused on how much more damage al Qaeda could have done on Sept. 11 "if they'd had a nuclear weapon and detonated it in the middle of one of our cities, or if they had unleashed . . . biological weapons of some kind, smallpox or anthrax." He then tied that to evidence found in Afghanistan of how al Qaeda leaders "have done everything they could to acquire those capabilities over the years."

But in October CIA Director George J. Tenet told Congress that Hussein would not give such weapons to terrorists unless he decided helping "terrorists in conducting a WMD [weapons of mass destruction] attack against the United States would be his last chance to exact vengeance by taking a large number of victims with him."

In his appearance Sunday, on NBC's "Meet the Press," the vice president argued that "we believe [Hussein] has, in fact, reconstituted nuclear weapons." But Cheney contradicted that assertion moments later, saying it was "only a matter of time before he acquires nuclear weapons." Both assertions were contradicted earlier by Mohamed ElBaradei, director general of the International Atomic Energy Agency, who reported that "there is no indication of resumed nuclear activities."

ElBaradei also contradicted Bush and other officials who argued that Iraq had tried to purchase high-strength aluminum tubes to use in centrifuges for uranium enrichment. The IAEA determined that Iraq did not plan to use imported aluminum tubes for enriching uranium and generating nuclear weapons. ElBaradei argued that the tubes were for conventional weapons and "it was highly unlikely" that the tubes could have been used to produce nuclear material.

Cheney on Sunday said ElBaradei was "wrong" about Iraq's nuclear program and questioned the IAEA's credibility.

Earlier this month, ElBaradei said information about Iraqi efforts to buy uranium were based on fabricated documents. Further investigation has found that top CIA officials had significant doubts about the veracity of the evidence, linking Iraq to efforts to purchase uranium for nuclear weapons from Niger, but the information ended up as fact in Bush's State of the Union address.

In another embarrassing episode for the administration, Secretary of State Colin L. Powell cited evidence about Iraq's weapons efforts that originally appeared in a British intelligence document. But it later emerged that the British report's evidence was based in part on academic papers and trade publications.

Sometimes information offered by Bush and his top officials is questioned by administration aides. In his March 6 news conference, Bush dismissed Iraq's destruction of its Al Samoud-2 missiles, saying they were being dismantled "even as [Hussein] has ordered the continued production of the very same type of missiles." But the only intelligence was electronic intercepts that had individuals talking about being able to build missiles in the future, according to a senior intelligence analyst.

Last month, Bush spoke about a liberated Iraq showing "the power of freedom to transform that vital region" and said "a new regime in Iraq would serve as a dramatic and inspiring example of freedom for other nations in the region." But a classified State Department report put together by the department's intelligence and research staff and delivered to Powell the same day as Bush's speech questioned that theory, arguing that history runs counter to it.

In his first major speech solely on the Iraqi threat, last October, Bush said, "Iraq possesses ballistic missiles with a likely range of hundreds of miles -- far enough to strike Saudi Arabia, Israel, Turkey and other nations -- in a region where more than 135,000 American civilians and service members live and work."

Inspectors have found that the Al Samoud-2 missiles can travel less than 200 miles -- not far enough to hit the targets Bush named. Iraq has not accounted for 14 medium-range Scud missiles from the 1991 Persian Gulf War, but the administration has not presented any evidence that they still exist.